AP BIOLOGY

Detailed explanation-1: -The extra chromosome 21 leads to the physical features and developmental challenges that can occur among people with Down syndrome.

Detailed explanation-2: -About 95 percent of the time, Down syndrome is caused by trisomy 21-the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.

Detailed explanation-3: -The classic form is the most common chromosomal disorder, in which there is one extra X chromosome resulting in the karyotype of 47, XXY [18]. Double aneuploidy was first described in a patient with both Down and Klinefelter (48, XXY, +21) syndromes.

Detailed explanation-4: -Chromosome 21 likely contains 200 to 300 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body.

Detailed explanation-5: -Down syndrome and trisomies 13 & 18 are chromosomal disorders that cause mental retardation and birth defects. Babies with Down syndrome have an extra chromosome #21 (trisomy 21) which causes mental retardation and various medical problems involving the heart, digestive tract, and/or other organ systems.